The present invention relates to control systems, and more particularly, to a method of incorporating independent feedforward control in a multivariable predictive controller.
In current control systems which includes feedforward control, a classical approach of handling the feedforward control is to include two controllers, a feedback (FB) controller and a feedforward (FF) controller, each controller having its own set of tuning parameters. In some systems currently available, a single feedback controller is utilized, and the errors of the feedback loop are combined with the disturbance error of the feedforward loop. A single corrective solution is generated by the FB controller which outputs moves to eliminate the feedback error and the feedforward disturbance effects. The single feedback controller has a single set of turning parameters thereby limiting the speed at which the effects of the disturbance can be canceled. If the speed of the controller is increased to increase the speed at which the disturbance is eliminated, the stability margin of the feedback controller is decreased. The feedforward control speed can be pushed faster than the feedback controller speed because the feedforward controller is an open loop control, i.e., the feedforward is inherently stable in its operation as long as the feedforward models are stable, whereas feedback control can become unstable if the control speed is pushed too hard. If the feedforward control is designated to go faster, there is no possibility of running into an unstable condition. If the user tunes the feedback controller to run faster, care must be taken to insure that the feedback controller doesn't get into an unstable condition. Thus the desire to have independent tuning parameters for the feedback and feedforward controller. A multivariable predictive controller, utilizing two control algorithms to perform a feedback and feedforward control in a single controller burns up too much CPU time. Thus, it is desired to provide a multivariable predictive controller such that the feedback controller combines the feedforward action with the feedback action and has separate independent tuning, one set of tuning parameters for the feedback control and one set of tuning parameters for the feedforward control.
Thus there is provided by the present invention a method of incorporating independent feedforward control in the feedback controller of a multivariable predictive controller.